1. Field of the Invention
The invention relates to a device for electromechanical stimulation and testing of hearing, comprising an electromechanical transducer for producing mechanical vibrations, and means for non-invasively transmitting the mechanical stimulation vibrations from the electromechanical transducer through the external auditory canal to at least approximately the center of the tympanic membrane (the umbo) and thus to the manubrium mallei of the ossicular chain.
2. Description of Related Art
Generally the audition of a person is tested in that a sound signal and thus an acoustical wave is presented to the test person in a monaural or binaural manner, and that the test person subjectively reacts on proper questions which are adequate to the respective purpose of the psychoacoustical test. Such electroacoustical devices are generally termed xe2x80x9caudiometerxe2x80x9d, and in most applications the test signal either is generated electronically (by analog or digital signal generators) or is derived from a suitable sound carrier (magnetic tape, compact disc and the like). These test signals then are acoustically presented to the test person, most frequently by loudspeakers under so-called called xe2x80x9cfree-fieldxe2x80x9d conditions, or by specifically calibrated measuring earphones. In particular cases, for example when an acoustically sealed volume in front of the tympanic membrane is required for the specific test, these acoustical signals are transmitted to the external auditory canal by short sound-conducting hoses and ear pieces.
Furthermore, objective hearing test methods are known (for example BERA: xe2x80x9cBrainstem Evoked Response Audiometryxe2x80x9d) in which acoustically evoked neuronal responses are picked up by skin electrodes and are correspondingly analyzed. Besides, the mobility of the middle ear is routinely determined by a method in which the middle ear is deflected by a pressure buildup within the external auditory canal, whilst one or more test tones are supplied by loudspeakers via transmission hoses, and in which pressure and velocity within the external auditory canal are measured by corresponding transducers (microphones and pressure meters). With the aid of these values, the acoustical impedance and changes thereof caused by pathological changes of the middle ear can be determined. The ability of triggering the stapedius reflex likewise can be measured when properly selecting the configuration of the envelope curve and the level.
In all these methods an acoustical signal is presented which, in a known manner, causes the tympanic membrane to mechanically oscillate, wherein these oscillations are transmitted via the ossicular chain of the middle ear to the inner ear and are converted there into a neuronal stimulation pattern which causes an auditory impression.
Furthermore, laser-audiometrical test devices are known (U.S. Pat. No. 6,007,494) in which dynamic movements of the tympanic membrane and of the manubrium mallei of the ossicular chain, respectively, can be optically, and thus in contactless manner, measured by a laser Doppler vibrometer even in the case of very small motion amplitudes near the auditory threshold in quiet.
In addition there are approaches to carry out examinations of the middle ear by direct contact with an electromechanical transducer (DE-A-31 21 429 and A. Thullen: xe2x80x9cKlinische Erfahrungen mit der Schallsonde nach Zxc3x6llnerxe2x80x9d, Medizinal-Markt, No. 12, 1956, pp. 444 to 445). In this case a sound probe is contacted with the middle ear, particularly invasively during middle ear operations. A device for pre-operative demonstration for implantable hearing systems and for psychoacoustical measurement of the auditory threshold in quiet by direct mechanical stimulation of the umbo is known from U.S. Pat. No. 5,833,626. This device comprises an electromechanical transducer for generating mechanical vibrations in the audio range, and a rigid mechanical coupling member for transmitting the mechanical vibrations, without surgery, through the external auditory canal in direct mechanical contact to the center of the tympanic membrane and thus to the manubrium mallei of the ossicular chain of the middle ear. Systems for non-invasively coupling such a device to the extra-corporal side of the tympanic membrane are described in U.S. Pat. No. 5,776,144.
German Patent No. 19 821 602 discloses a vibration measuring head comprising an electromechanical transducer, which head permits, exclusively in resonance operation, an evaluation of the movability of the vibratorily movable elements of the middle ear structure coupled to the actoric side of the transducer by means of a second measuring coil because the dampening of the system by the middle ear structure coupled thereto is represented by a variation of the voltage generated by this coil.
Especially in the described objective hearing testing methods (for example, BERA) there are, however, some disadvantages in the type of acoustic excitation, such as for example the magnetic fields generated by the usually used electrodynamic or electromagnetic headphones. These magnetic (interference) fields lead to problems in pre-processing and analysis of the evoked potentials which are electrically derived from the skin surface of the head and which can be in the nV range. For acoustic signals monaurally presented supraliminally at medium to high sound levels the problem of xe2x80x9coverhearingxe2x80x9d of the contralateral ear, which is not being tested, due to the acoustic sound emission of the headphone or by body sound (bone conduction) continues to occur, which leads to the necessity of acoustic masking of the contralateral ear. This effect is undesirable in many psychoacoustic situations, but inevitable. A disadvantage in using the device known from German Patent No. 198 21 602 particularly is that a broad-band measurement is impossible because the method applied is based on resonance effects of the transducer. Thus a true determination of the mechanical impedance of the middle ear structure coupled to the device is not possible, particularly when measurements in different spectral regions are desired. However, measurements in different spectral regions, at least within the main speech frequency range, are of substantial importance when precise information about the pathological condition of the middle ear or even of the inner ear, which is mechanically coupled thereto, is to be obtained.
A primary object of the present invention is to provide for a device for electromechanical stimulation and testing of hearing which permits in a particularly reliable manner a non-invasive objective testing of a person""s auditory capacity.
According to the invention, this object basically is achieved by a device for electromechanical stimulation and testing of hearing, comprising an electromechanical transducer for producing mechanical stimulation vibrations; means for non-invasively transmitting the mechanical stimulation vibrations from the electromechanical transducer through the external auditory canal to at least approximately the center of the tympanic membrane (the umbo) and thus to the manubrium mallei of the ossicular chain; and an impedance measuring system for measuring the mechanical impedance of a biological load structure which is coupled to the transducer.
The presently used electromechanical transducer basically may be designed in conformity with any known transducer principle. Thus, this transducer particularly may be an electromagnetic, electrodynamic, magnetostrictive, dielectric and particularly piezoelectric transducer.
In conformity with one embodiment of the invention, the impedance measuring system may comprise means for measuring the electrical input impedance of the electromechanical transducer which is coupled to the biological load structure. The data for magnitude and phase of this electrical input impedance reflect the load components coupled to the device because these load components, transformed via the electromechanical coupling of the transducer, appear on the electrical side and thus can be measured.
Preferably, the electromechanical transducer is driven by a driver unit to which the transducer is connected via a measuring resistance across which a measuring voltage proportional to a transducer current is dropped, and a measuring amplifier is provided which has applied thereto, as input signals, said measuring voltage and a transducer terminal voltage. In order to preclude a corruption of the measurements, the voltage drop across the measuring resistance preferably is taken off in a floating and high impedance manner, and the measuring resistance advantageously is dimensioned such that the sum of the resistance value of the measuring resistance and of the absolute value of the complex electrical input impedance of the electromechanical transducer coupled to the biological load structure is large with respect to the internal resistance of the driver unit. Furthermore, preferably digital, means are provided for forming the quotient of the transducer terminal voltage and the transducer current.
According to an alternate embodiment of the invention the impedance measuring means, however, also may be designed for direct measurement of the mechanical impedance of the biological load structure coupled to the electromechanical transducer, and such impedance measuring means may be integrated into the transducer at the actoric output side thereof. Preferably, the impedance measuring means is designed for generating measuring signals which are at least approximately proportional as to magnitude and phase to the force acting on the biological load structure and the velocity of the transmitting means, respectively. In such a case, the system advantageously further includes a two-channel measuring amplifier and, preferably digital, means for providing the quotient of the measuring signal corresponding to the force acting on the biological load structure and of the measuring signal corresponding to the velocity of the of the transmitting means.
In the case of the direct impedance measurement the electromechanical transducer and the impedance measuring means may be disposed within a common housing which optionally also receives the measuring amplifier.
The described impedance measurements by no means are restricted to a single measuring frequency or to a single measuring level. Rather, advantageously for indirect as well as for direct measurement of the mechanical impedance of the biological load structure, preferably digital, means are provided for measuring the mechanical impedance of the biological load structure coupled to the electromechanical transducer as a function of the frequency and/or of the level of the stimulation signal delivered by the transducer. Measurements extending over the entire transmission frequency range and the entire stimulation level range are particularly suited to gain important detailed information about linear and particularly non-linear variations of the middle ear and/or the inner ear. Thus, for example, it may be expected that a mechanical non-linearity of the middle ear caused for example by a partial luxation of a middle ear ossicle, can be detected by varying the electrical level during the impedance measurement.
When a level analysis as to non-linearities over the entire useful level range is dispensed with, and when the measuring system is of a sufficiently sensitive and low-noise type, the measurement of the electrical transducer impedance also may be carried out below the individual patient""s auditory threshold in quiet in order to avoid any disturbance of the patient by the measuring signals.
In conformity with a further embodiment of the invention, preferably digital, means may be provided for detecting the spectral distribution of resonance frequencies in the course of the impedance measured as a function of the frequency of the stimulation signal, and also means for detecting the difference between the measured impedance values occurring at the resonance frequencies. This difference gives information as to the mechanical oscillation Q-values.
Preferably, a passive coupling element is provided for coupling the electromechanical transducer to the umbo. The impedance measuring system for direct measurement of the mechanical impedance may be incorporated into this coupling element.
In conformity with an embodiment of the invention the electromechanical transducer may be disposed within a housing the geometrical dimensions of which are selected such that, upon placing the transducer in an entrance region of the external auditory canal, the person carrying out the test, even when using a microscope, has an unobstructed view onto the actor end of the coupling element mechanically contacting the umbo.
The coupling rod preferably is designed in a manner which allows to easily manually curve or bend the coupling rod, whereby the coupling rod can be simply adapted to individual geometrical shapes of the external auditory canal. Furthermore, the coupling element advantageously is not mechanically fixedly, but rather detachably connected to the electromechanical transducer, preferably by means of plug-type connector means. This, for example, permits the utilization of various different coupling elements which, for hygienical reasons, can be designed as easily exchangeable single-use articles.
Preferably, the first mechanical resonance frequency of a combination comprised of the electromechanical transducer and the mechanical coupling element is disposed at the upper end of the spectral transmission range of at least 10.000 cps. Such a broad-band design provides for short transient periods.
According to a further embodiment of the invention the electromechanical transducer has a mechanical source impedance which, in the entire spectral transmission range, is distinctly higher than the mechanical load impedance defined by a biological system comprising the tympanic membrane, the ossicular chain and the inner ear. This provides for an impression of the deflection of the actor end of the coupling element which is independent of individual variations of the biological load impedance.
Preferably the electromechanical transducer and a driver unit driving the latter are designed such that the transducer and the mechanical coupling element, with the ossicular chain being coupled thereto, in the entire spectral transmission range, have maximum amplitudes of deflection in the region from 1 to 5 micrometers corresponding to equivalent sound pressure levels from 120 to 140 dB SPL.
Advantageously the electromechanical transducer is encapsulated for minimizing the sound signal emitted by the oscillating transducer structures. Thereby an acoustic masking of the contralateral ear which is not tested frequently can be dispensed with even at high stimulation levels.
The device may be provided in duplicate for simultaneously stimulating and testing both ears of a test person.
These and further objects, features and advantages of the present invention will become apparent from the following description when taken in connection with the accompanying drawings which, for purposes of illustration only, shows several embodiments in accordance with the present invention.